Laser oscillator

ABSTRACT

A laser oscillator which uses a granular laser gain medium to secure a large gain volume and is capable of suppressing resonator loss caused by granular-boundary scattering. A suspension (1), which is obtained by dispersing a number of dielectric grains doped with a luminous element in fluid having refractive index matched with that of the dielectric grains, is filled in a cell (6). The cell (6) filled with the suspension (1) is disposed in an optical resonator constituted by a rear mirror (3) and an output mirror (4). When the cell (6) is irradiated with excitation light (2), the dielectric grains in the suspension (1) are subjected to laser pumping and a laser beam (5) is output. Since the dielectric grains coexist with the refractive index-matched fluid, the granular-boundary scattering is suppressed, making it possible to provide a low-loss optical resonator. Dielectric grains with a slightly greater diameter may alternatively be distributed in filling liquid which is refractive index-matched fluid.

TECHNICAL FIELD

The present invention relates to a laser oscillator using a granularsolid material as a gain medium, and more particularly to a laseroscillator using a number of dielectric grains admixed with lanthanideseries or the like, as the gain medium.

BACKGROUND ART

As a typical material used as a gain medium for a solid-state laseroscillator, there is known a dielectric bulk of, for example, Y₃ Al₅ O₁₂doped with a luminous element such as lanthanide series. The form ofsuch a bulk is in general single crystal or amorphous. This is because,if a bulk material of some other form (mass of polycrystalline orcrystalline grains) is placed in an optical resonator, the resonatorloss increases due to light scattering occurring inside the bulk, makingit impossible to obtain a sufficient laser output.

In order to further increase the output of a solid-state laser, it isnecessary to enlarge the gain medium of the laser oscillator in size toincrease the gain length or to increase the percentage of a luminouselement (dopant concentration) doped in the gain medium. However, usinga large-sized single crystal is not advantageous in terms of cost, andpresent-day techniques of single crystal growth place restrictions onincreasing the doping concentration of the luminous element.

If an amorphous material doped with a luminous element such aslanthanide series is used as the gain medium, then it is relatively easyto increase the size of the gain medium and to increase the dopingconcentration of the luminous element. Amorphous materials in generalare, however, low in heat conductivity and difficulty arises inefficiently dissipating heat generated during high-output operation ofthe laser oscillator, possibly causing thermal breakdown of thematerials. It is, therefore, difficult to use an amorphous materialdoped with a luminous element such as lanthanide series, as the gainmedium for a high-output laser.

Thus, there is a difficulty in increasing the output of a laseroscillator by using, as its gain medium, a material doped with aluminous element such as lanthanide series and having the form of singlecrystal or amorphous bulk. To resolve the situation, attempts have beenmade to use as the gain medium of a laser oscillator a material dopedwith a luminous element such as lanthanide series and having the form ofnumerous dielectric grains.

These attempts are based on the idea that dielectric grains consistingof high-quality single crystals can be relatively easily produced at lowcost, and since a large heat radiating surface area is ensured even ifthe dielectric grains are amorphous, the possibility of thermalbreakdown lessens and increase in the output of a laser oscillator canbe expected.

However, where dielectric grains are used as the gain medium, increasedgain volume for higher output of a laser oscillator constitutes anadditional cause of the resonator loss, impeding increase of the outputof the laser oscillator. Specifically, in cases where dielectric grainsare used as the gain medium, if the density of dielectric grains isincreased to thereby increase the gain volume, then granular-boundaryscattering of a laser beam occurring between the dielectric grainsintensifies and thus the resonator loss increases, making it difficultto increase the laser oscillator output in practice.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a low-cost,high-output, high-efficiency laser oscillator using, as a gain mediumthereof, dielectric grains admixed with a variety of laser luminouselements.

According to the present invention, a large number of dielectric grainsdoped with a luminous element are distributed in a medium havingrefractive index matched with that of the dielectric grains, and thedielectric grains and the refractive index-matched medium are arrangedwithin an optical resonator as a laser gain medium. The "refractiveindex-matched medium" used herein denotes a medium having suchrefractive index that the difference in refractive index between thedielectric grains doped with a laser luminous element and the medium isrelatively small. Provided the refractive index of the former(dielectric grains doped with a laser luminous element) is n₁, therefractive index of the latter (refractive index-matched medium) is n₂,and the absolute difference in refractive index between the two is n=|n₁-n₂ |, the relative refractive index difference r_(n) =n/n₁, shoulddesirably be as close to zero as possible. When actually selectingmaterials, an upper limit of, for example, r_(n) ≦0.45, may be adoptedas a criterion.

In the case where the numerous dielectric grains doped with a luminouselement have so small a diameter (e.g., 1 mm or less) that they can besuspended as suspended grains in fluid having refractive index matchedwith that of the dielectric grains, the numerous dielectric grains andthe fluid may be contained in the cell in the form of suspension, andthe cell is disposed in the optical resonator to constitute a laseroscillator.

On the other hand, where the numerous dielectric grains doped with aluminous element have such a large diameter that they can not besuspended in the refractive index-matched fluid, the numerous dielectricgrains are contained in the cell in a manner such that they are immersedin filling liquid having refractive index matched with that of thedielectric grains, and the cell is disposed in the optical resonator toconstitute a laser oscillator.

Preferably, the cell is connected to a circulation system forcirculating the suspension or the filling liquid via a path extendingoutside of the cell, to thereby cool the suspension or the fillingliquid. In this case, the suspension or the filling liquid may be causedto flow within the cell either in a direction substantially parallel toor in a direction traversing the optical axis of the optical resonator.The former is advantageous in that the structure can be simplified,while the latter is advantageous in that the cooling can be efficientlyperformed.

The cell may have a shape of either slab or rod, as in the case of asolid-state laser using a bulk of similar composition (luminouselement+dielectric). It is also possible to machine input/output windowsat opposite ends of the cell so as to be inclined at Brewster's angle.

As materials of the dielectric grains to be doped with a luminouselement, Al₂ O₃, YAG, YAlO₃, YVO₄, S-VAP, GdVO₄, GLF, BYF, KYF, KLYF,KLGF, GGG, LOS, BGO, etc. can be given as specific examples. Theluminous element used as a dopant may be Cr or Ti, in addition tolanthanide series.

A large number of dielectric grains doped with a laser luminous elementare distributed in a medium having refractive index matched with that ofthe dielectric grains and the medium having the dielectric grainsdistributed therein is used as the laser gain medium, whereby theconventional problem of resonator loss can be solved for the reasonexplained below, referring to FIGS. 1 and 2.

FIGS. 1 and 2 illustrate light scattering and refraction observed,respectively, in the case where there is a large difference inrefractive index between a dielectric grain and a medium surrounding thesame, and in the case where the refractive index difference is small(the dielectric grain is surrounded by refractive index-matched fluid).

As shown in FIGS. 1 and 2, in general, when incident light 7 falls uponthe surface of a dielectric grain 10 (refractive index: n₁) from amedium (refractive index: n₂ ; n₂ ≠n₁) surrounding the dielectric grain10, part of the light is propagated through the grain (internallypropagated light 71) while the remaining light is reflected (reflectedlight 72). The internally propagated light 71 then impinges on thesurface of the dielectric grain 10 from inside and is split intotransmitted light 73 and internally reflected light 74. Most of theinternally reflected light 74 travels to the outside of the dielectricgrain 10 as retrograde light 75.

As is well known in the basic theory of optics, where there is a largedifference between the refractive index n₁ of the dielectric grain andthe refractive index n₂ of the medium surrounding the grain (see FIG.1), the rate of occurrence of the reflected light 72 or the internallyreflected light 74 is high. Also, the direction of the incident light 7and the direction of propagation of the transmitted light 73 are greatlydifferent from each other. These factors are causes of increase inso-called granular-boundary scattering components. Since the refractiveindex of the dielectric grain is in general considerably greater than"1" (for example, the refractive index of Al₂ O₃ is approximately 1.77),if a granular dielectric is arranged within the optical resonator in thesame manner as in the case of using a dielectric bulk, then theaforementioned intergranular scattering occurs intensely, increasing theloss.

On the other hand, in the case where the refractive index n₁ of thedielectric grain and the refractive index n₂ of the medium surroundingthe grain are close to each other, that is, where the dielectric grainis suspended or immersed in the refractive index-matched fluid (see FIG.2), the rate of occurrence of the reflected light 72 or the internallyreflected light 74 is low and the directions of propagation of theincident light 7 and the transmitted light 73 are nearly parallel toeach other. These factors each contribute to reducing the intergranularscattering component. As a result, the loss of the optical resonatorlessens, permitting efficient laser oscillation.

Thus, in the laser oscillator according to the present invention, alarge number of dielectric grains are used to thereby secure asufficient laser gain volume, and the dielectric grains are distributedin the refractive index-matched fluid, so that the laser resonator losscaused due to light scattering within the gain medium can be lessened.It is, therefore, possible to provide a high-efficiency, high-outputlaser oscillator that can be produced more easily than ever.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating light scattering and refractionobserved in the case where there is a large difference in refractiveindex between a dielectric grain and a medium surrounding the grain;

FIG. 2 is a diagram illustrating light scattering and refractionobserved in the case where a dielectric grain is surrounded by arefractive index-matched fluid;

FIG. 3 is a sectional view showing the arrangement of a principal partof a laser oscillator according to Embodiment [1];

FIG. 4 is a sectional view showing the arrangement of a principal partof a laser oscillator according to Embodiment [2];

FIG. 5 is a schematic view showing the arrangement of a principal partof a laser oscillator according to Embodiment [3];

FIG. 6 is a sectional view showing the arrangement of a principal partof a laser oscillator according to Embodiment [4];

FIG. 7 is a sectional view showing the arrangement of a principal partof a laser oscillator according to Embodiment [5]; and

FIG. 8 is a schematic view showing the arrangement of a principal partof a laser oscillator according to Embodiment [6].

BEST MODE OF CARRYING OUT THE INVENTION

Embodiments [1] to [6] of the present invention will be hereinafterdescribed referring to FIGS. 3 through 8.

Embodiment [1]

FIG. 3 illustrates, in section, the arrangement of a principal part of alaser oscillator according to Embodiment [1]. In FIG. 3, a gain mediumcharacterizing the present invention is embodied as a suspension 1. Thesuspension 1 is obtained by dispersing a large number of fine dielectricgrains (e.g., of a grain diameter of 1 mm or less), which have beendoped with lanthanide series or the like, in refractive index-matchedfluid so as to be suspended therein, the suspension being filled in acell 6. The cell 6 filled with the suspension 1 is arranged within anoptical resonator constituted by a rear mirror (total reflection mirror)3 and an output mirror (partial transmission mirror) 4.

The illustrated cell 6 is in the form of a slab whose opposite end facesare machined to be inclined at Brewster's angle, but the cell mayalternatively take the form of a rod. Further, the machining of the cellto obtain end faces inclined at Brewster's angle may in some cases beomitted. This applies not only to this embodiment but also toEmbodiments [2] through [6].

One example of the dopant used is Ti, and a dielectric used incombination with this is, for example, Al₂ O₃. Other gain media than thecombination Ti:Al₂ O₃ include, for example, Yb, Cr, Nd, Er, Hf or thelike as the dopant, and YAG, YAlO₃, YVO₄, S-VAP, GdVO₄, GLF, BYF, KYF,KLYF, KLGF, GGG, LOS, BGO or the like as the dielectric.

Where Ti:Al₂ O₃ is used as the gain medium, the refractive index of Al₂O₃ is approximately 1.77 and this value does not change much even if Al₂O₃ is doped with Ti (in general, the dopant concentration is verysmall); therefore, for the suspending medium, liquid may be used whichis obtained by diluting a saturated solution, which contains a mixtureof methylene iodide, yellow phosphorus and sulfur in the ratio of 1:8:1,for example, with methylene iodide to obtain an approximately 1/11dilution.

In the case where the suspension 1 contains Nd:YAG, for example, and notTi:Al₂ O₃, the dilution may be about 1/4. When selecting the suspendingmedium, care must be taken that the refractive index of the suspendingmedium is as close to that of the dielectric grains as possible (i.e.,the relative refractive index difference r_(n) between the suspendingmedium and the dielectric grains is close to zero), and for materialsthat fulfill this condition to an almost identical degree, thosematerials which have as small an absorbance as possible with respect tothe wavelength of the laser as well as to the wavelength of excitationlight should preferably be selected.

When the cell 6, which is filled with the suspension 1 selected takingthe above into account, is irradiated with excitation light 2, thenumerous dielectric grains doped with lanthanide series or the like andsuspended in the suspension 1 are subjected to laser pumping and laseroscillation takes place inside the optical resonator constituted by therear mirror 3 and the output mirror 4, so that a laser beam 5 is outputfrom the output mirror 4.

The laser beam 5 is propagated through the suspension in the cell 6, andsince the suspending medium is selected such that change in therefractive index at its interface with the dielectric grains is small,neither scattering nor refraction of the laser beam is noticeable whenthe beam passes across the interface with the dielectric grains, asmentioned above, never causing a large resonator loss. Thus, accordingto this embodiment, a laser oscillator is provided in which a largenumber of dielectric grains doped with lanthanide series or the like aredispersed in the refractive index-matched medium and which has a smallresonator loss. Although the illustration of FIG. 1 is such that theexcitation light 2 is radiated from outside the cell 6, the source ofthe excitation light may be located either outside or inside of the cell6. In the case where the excitation light source is arranged outside thecell 6, however, part of the cell where the excitation light is to beintroduced, in addition to windows at the opposite ends thereof throughwhich the laser beam is transmitted, needs to be transparent. This isthe case with the other embodiments described below.

Embodiment [2]

FIG. 4 illustrates the arrangement of a principal part of a laseroscillator according to Embodiment [2] in a manner similar to FIG. 3.Compared with the laser oscillator of Embodiment [1] shown in FIG. 3,the laser oscillator of this embodiment additionally includes asuspension circulating/cooling device 11. Similar to an ordinary fluidcirculating/cooling device, the suspension circulating/cooling device 11to be used may be of the type equipped with a circulating pump forcirculating the suspension 1 and a cooler for forcibly cooling thesuspension 1. If, however, the suspension 1 can be sufficiently cooledwhile being circulated, the cooler may be omitted from the device.

The arrangement of the laser oscillator except the parts associated withthe suspension circulating/cooling device 11 is identical with that ofthe laser oscillator of Embodiment [1]. Namely, the suspension 1 isobtained by dispersing a number of fine dielectric grains (e.g., of agrain diameter of 1 mm or less) as suspended grains, which have beendoped with lanthanide series or the like, in the refractiveindex-matched fluid, and is enclosed within a circulation path includingthe cell 6 and the suspension circulating/cooling device 11 to flowtherethrough for circulation. The cell 6, which is part of thecirculation path for the suspension 1, is disposed in the opticalresonator constituted by the rear mirror (total reflection mirror) 3 andthe output mirror (partial transmission mirror) 4, like Embodiment [1].The composition of the suspension 1 is identical with that explainedwith reference to Embodiment [1], and therefore, description thereof isomitted.

While the suspension circulating/cooling device 11 of the laseroscillator shown in FIG. 4 is operated, the cell 6 is irradiated withthe excitation light 2, whereupon the numerous dielectric grains dopedwith lanthanoide series or the like and suspended in the suspension 1are subjected to laser pumping and laser oscillation takes place insidethe optical resonator constituted by the rear mirror 3 and the outputmirror 4, so that a laser beam 5 is output from the output mirror 4.

Since, in general, the energy efficiency of laser oscillation is not100%, part of energy of the excitation light 2 unavoidably converts intothermal energy. As the temperatures inside and outside the cell 6 risedue to the thermal energy, the resonator loss occurs due to deformationof the cell 6 or dispersion occurs due to the Doppler effect,deteriorating the output characteristics of the laser oscillator.According to this embodiment, the suspension 1 is circulated in such amanner that it flows away from the optical resonator to a location onone side of the same and passes through the suspensioncirculating/cooling device 11, and therefore, the temperatures insideand outside the cell 6 are prevented from rising, thereby avoidingdeterioration in the output characteristics.

Where the suspension 1 is caused to flow within the cell 6 in adirection parallel to the optical axis of the optical resonator as inthe illustrated arrangement, the arrangement of parts surrounding thelaser resonator can advantageously be kept simple even though thesuspension circulating/cooling device 11 is incorporated, as comparedwith the case where the suspension flows in a direction traversing(e.g., perpendicular to) the optical axis of the optical resonator. Itshould be noted, however, that the effect of eliminating temperaturegradient which is liable to occur in a region from the vicinity of theaxis toward the periphery of the optical resonator somewhat lessens,compared with the case where the suspension 1 flows in a directiontraversing the optical axis of the optical resonator (see the followingexplanation of Embodiment [3]).

Embodiment [3]

FIG. 5 is a schematic diagram illustrating the arrangement of aprincipal part of a laser oscillator according to Embodiment [3]. In thelaser oscillator of this embodiment, a large number of fine dielectricgrains (e.g., of a grain diameter of 1 mm or less) doped with lanthanideseries or the like are dispersed as suspended grains in the refractiveindex-matched fluid and the suspension 1 thus obtained is circulated, asin Embodiment [2] shown in FIG. 4; however, Embodiment [3] differs fromEmbodiment [2] in that the suspension 1 is caused to flow within thecell 6 in a direction perpendicular to the optical axis of the opticalresonator.

In this case, a smooth flow of the suspension 1 perpendicular to theoptical axis of the optical resonator needs to be formed within the cell6 by means of, for example, an outlet port 61 constituted by a taperedpipe portion 61a and a flat pipe portion 61b and an inlet port 62constituted by a tapered pipe portion 62a and a flat pipe portion 62b,both of the ports 61 and 62 being connected to the cell 6. Consequently,the arrangement of parts surrounding the laser resonator is rathercomplicated, as compared with Embodiment [2] described above.Nevertheless, since the suspension 1 flows in a direction across theoptical axis of the optical resonator, temperature gradient canadvantageously be prevented from occurring in a region from the vicinityof the axis toward the periphery of the optical resonator.

Further, in the case where the suspension 1 is caused to flow within thecell 6 in a direction perpendicular to the optical axis of the opticalresonator as in this embodiment, it is easy to make the cross-sectionalarea of the circulation passage relatively large, and accordingly, thecirculation quantity of the suspension 1 can be increased even if thecirculation velocity is low, thus facilitating efficient heat removal.In FIG. 5, the suspension circulating/cooling device arranged so as toconnect the outlet port 61 to the inlet port 62 is omitted.

Embodiment [4]

FIG. 6 is a sectional view similar to FIG. 1 and shows the arrangementof a principal part of a laser oscillator according to Embodiment [4].In the laser oscillator of this embodiment, the dielectric grains in thelaser oscillator according to Embodiment [1] shown in FIG. 3, whichcomprise fine grains dispersed and suspended in the refractiveindex-matched fluid, are replaced by a medium of relatively large size(e.g., 1 mm to several centimeters in grain diameter) immersed anddistributed in the refractive index-matched fluid.

In this embodiment, a large number of dielectric grains 13 doped withlanthanoide series or the like are enclosed within the cell 6 in amanner such that the grains are immersed in filling liquid 12 which isrefractive index-matched fluid, and the cell is disposed inside theoptical resonator constituted by the rear mirror 3 and the output mirror4. Namely, this embodiment uses, as the laser gain medium, thedielectric grains 13 having so large a grain diameter that they cannotbe suspended in the refractive index-matched fluid, and is similar inarrangement to Embodiment [1] except that the dielectric grains usedhave a larger diameter.

Specifically, the cell 6 is in the form of a slab whose opposite endfaces are machined so as to be inclined at Brewster's angle. Thecomposition of the dielectric grains 13, which are accommodated in thecell 6 together with the filling liquid 12, is identical with that usedin Embodiments [1] to [3]. Namely, the grains to be used may be made ofa material containing a dielectric, such as Al₂ O₃,YAG, YAlO₃, YVO₄,S-VAP, GdVO₄, GLF, BYF, KYF, KLYF, KLGF, GGG, LOS, BGO or the like,admixed with Ti, Yb, Cr, Nd, Er, Hf or the like as a dopant.

The composition of the filling liquid (refractive index-matched fluid)12 may also be identical with that used in Embodiments [1] to [3]. Forexample, where dielectric grains containing Al₂ O₃ as a main componentare used, the refractive index thereof is approximately 1.77; therefore,for the filling liquid 12, liquid may be used which is obtained bydiluting a saturated solution, which contains a mixture of methyleneiodide, yellow phosphorus and sulfur in the ratio of 1:8:1, for example,with methylene iodide to obtain an approximately 1/11 dilution. In thecase where the dopant-dielectric combination used is Nd:YAG, the aboveliquid may be diluted to obtain an approximately 1/4 dilution.

When selecting the filling liquid 12, similar care to that required whenselecting the suspending medium should be taken. Specifically, as thefilling liquid 12, a material of which the refractive index is as closeto that of the dielectric grains as possible and at the same time whichhas as small an absorptance as possible with respect to the wavelengthof the laser as well as to the wavelength of the excitation light shouldpreferably be selected.

When the cell 6, which is filled with the dielectric grains 13 and thefilling liquid 12 selected taking the above into account, is irradiatedwith the excitation light 2, the numerous dielectric grains 13 dopedwith lanthanide series or the like are subjected to laser pumping andlaser oscillation takes place inside the optical resonator constitutedby the rear mirror 3 and the output mirror 4, so that a laser beam 5 isoutput from the output mirror 4.

The laser beam 5 encounters the dielectric grains 13 one after anotherwhile being propagated through the filling liquid 12 in the cell 6, andsince the filling liquid 12 is selected such that change in therefractive index at its interface with the dielectric grains 13 issmall, neither scattering nor refraction of the laser beam is noticeablewhen the beam passes across the interfaces between the filling liquid 12and the dielectric grains 13, as mentioned above, never causing a largeresonator loss. Thus, also according to this embodiment, a laseroscillator is provided in which a large number of dielectric grainsdoped with lanthanoide series or the like are distributed in therefractive index-matched medium and which has a small resonator loss.

Embodiment [5]

FIG. 7 illustrates the arrangement of a principal part of a laseroscillator according to Embodiment [5] in a manner similar to FIG. 1 or4. Compared with the laser oscillator of Embodiment [4] shown in FIG. 6,the laser oscillator of this embodiment additionally includes a fillingliquid circulating/cooling device 14. As the filling liquidcirculating/cooling device 14, an ordinary fluid circulating/coolingdevice can be used to circulate and cool the filling liquid 12, and maybe of the type equipped with a circulating pump and a cooler forforcibly cooling the filling liquid 12. If, however, the filling liquid12 can be sufficiently cooled while being circulated as in the case ofcirculating the suspension n Embodiment [2] or [3], the cooler may beomitted from the device.

The laser oscillator of this embodiment is identical in arrangement withthat of Embodiment [4] except for the parts associated with the fillingliquid circulating/cooling device 14. Specifically, a number ofdielectric grains of relatively large size (e.g., 1 mm to severalcentimeters in grain diameter), which have been doped with lanthanideseries or the like, are immersed and distributed in the filling liquid12 which is refractive index-matched fluid. Although this embodiment issimilar in arrangement to Embodiment [2], the laser gain medium used isnot suspended grains, but immersed grains distributed in the fillingliquid (refractive index-matched fluid) 12, and therefore, thisembodiment differs from Embodiment [2] in that only the filling liquid(refractive index-matched fluid) 12 is allowed to flow through thecirculation passage which extends outside of the optical resonator. Inthe case where the diameter of the dielectric grains 13 is equal to orsmaller than that of filling liquid outlet and inlet at opposite ends ofthe cell 6, a suitable grid member or the like is desirably provided sothat the dielectric grains 13 can be prevented from flowing out of thecell 6.

Like Embodiment [4], the cell 6, which forms part of the circulationpath for the filling liquid 12, is in the form of a slab whose end facesare machined so as to be inclined at Brewster's angle, and is disposedin the optical resonator constituted by the rear mirror (totalreflection mirror) 3 and the output mirror (partial transmission mirror)4. The compositions of the dielectric grains 13 and the filling liquid12 are identical with those used in Embodiment [4], and therefore,description thereof is omitted.

While the filling liquid circulating/cooling device 14 of the laseroscillator shown in FIG. 7 is operated, the cell 6 is irradiated withthe excitation light 2, whereupon the numerous dielectric grains 13doped with lanthanoide series or the like and immersed in the fillingliquid 12 are subjected to laser pumping and laser oscillation takesplace inside the optical resonator constituted by the rear mirror 3 andthe output mirror 4, so that a laser beam 5 is output from the outputmirror 4.

The thermal energy generated within the cell 6 is conveyed by thefilling liquid 12 which flows inside the cell 6 in the direction of theoptical axis thereof and then to the outside of the optical resonatorfrom one end portion of the cell 6. The filling liquid thereafter passesthrough the filling liquid circulating/cooling device 14 and, after heatis dissipated satisfactorily, flows again into the cell 6 from the otherend portion thereof. Consequently, the resonator loss which isattributable to deformation of the cell 6 due to increase intemperature, dispersion due to the Doppler effect, etc. can beprevented, thereby avoiding deterioration in the output characteristicsof the laser oscillator.

In the case where the filling liquid 12 is caused to flow within thecell 6 in a direction parallel to the optical axis of the opticalresonator as in this arrangement, the arrangement of parts surroundingthe laser resonator can advantageously be kept simple even though thefilling liquid circulating/cooling device 14 is incorporated, ascompared with the case where the filling liquid flows in a directionacross (e.g., perpendicular to) the optical axis of the opticalresonator. It should be noted, however, that like Embodiment [2], theeffect of eliminating the temperature gradient which is liable to occurin a region from the vicinity of the axis toward the periphery of theoptical resonator somewhat lessens, as compared with the case where thefilling liquid 12 flows in a direction across the optical axis of theoptical resonator (see the following explanation of Embodiment [6]).

Embodiment [6]

FIG. 8 schematically illustrates the arrangement of a principal part ofa laser oscillator according to Embodiment [6]. In the laser oscillatorof this embodiment, a number of dielectric grains of relatively largesize (e.g., of 1 mm to several centimeters in grain diameter), whichhave been doped with lanthanide series or the like, are immersed anddistributed in the filling liquid 12 which is refractive index-matchedfluid, and the filling liquid 12 is circulated so as to flow outside ofthe optical resonator, as in Embodiment [5] shown in FIG. 7; however,Embodiment [6] differs from Embodiment [5] in that the filling liquid 12is caused to flow within the cell 6 in a direction perpendicular to theoptical axis of the optical resonator.

In this case, like Embodiment [3] (see FIG. 5), a smooth flow of thefilling liquid 12 perpendicular to the optical axis of the opticalresonator needs to be formed within the cell 6 by means of, for example,an outlet port 61 constituted by a tapered pipe portion 61a and a flatpipe portion 61b and an inlet port 62 constituted by a tapered pipeportion 62a and a flat pipe portion 62b, both of the ports 61 and 62being connected to the cell 6. Also, where the diameter of thedielectric grains 13 is equal to or smaller than the maximum thickness(axis-side thickness) of the tapered portions 61a and 62a close to thecell 6, a suitable grid member or the like is desirably arranged in sucha manner that it stretches over nearly the entire length of the cell 6,to thereby prevent the dielectric grains 13 from flowing out of the cell6.

Thus, the arrangement of parts surrounding the laser resonator is rathercomplicated, as compared with Embodiment [5] described above.Nevertheless, since the filling liquid 12 flows in a direction acrossthe optical axis of the optical resonator, temperature gradient canadvantageously be prevented from occurring in a region from the vicinityof the axis toward the periphery of the optical resonator.

Further, in the case where the filling liquid 12 is caused to flowwithin the cell 6 in a direction perpendicular to the optical axis ofthe optical resonator as in this embodiment, it is easy to make thecross-sectional area of the circulation passage relatively large, andaccordingly, the circulation quantity of the filling liquid 12 can beincreased even if the circulation velocity is low, thus facilitatingefficient heat removal. In FIG. 8, the filling liquidcirculating/cooling device arranged so as to connect the outlet port 61to the inlet port 62 is omitted.

In each of Embodiments [1] through [6] described above, a materialobtained by doping a granular dielectric of small size with lanthanoideseries or the like is used as the laser gain medium, and the dielectricmaterial used should preferably be in the form of single crystal oramorphous (polycrystalline form is not preferred because of largeinternal scattering).

As described above, the present invention uses a granular dielectricadmixed with a luminous element as the laser gain medium to secure alarge gain volume, and also uses a refractive index-matched fluid tokeep the resonator loss, which is caused by light scattering, at a lowlevel. Accordingly, the present invention can provide a laser oscillatorhaving efficiency and output performance comparable to a solid-statelaser in general use which uses, as the gain medium, a solid materialadmixed with a rare-earth element or the like.

Generally, producing small-sized dielectric materials of high quality ismuch easier and less expensive than producing large-sized bulks, andthis advantage is marked especially in cases where single-crystaldielectric is used. Also, the refractive index-matched fluid can beutilized as heat dissipating means as well, and thus deterioration inoscillation characteristics due to rise in temperature can be easilysuppressed.

According to the present invention, therefore, it is possible to providea laser oscillator which exhibits performance substantially equal tothat of solid-state lasers using a laser gain medium of similarcomposition and yet can be produced much more easily and lessexpensively than such conventional solid-state lasers.

We claim:
 1. A laser oscillator comprising:a large number of dielectricgrains having a first refractive index and being doped with an elementof the periodic table which has luminous properties; fluid having asecond refractive index matched with the first refractive index of saiddielectric grains; and an optical resonator; said dielectric grains andsaid fluid being positioned in said optical resonator as a laser gainmedium, in the path of light passing through the optical resonator. 2.The laser oscillator according to claim 1, wherein said dielectricgrains contains one of Al₂ O₃, YAG, YAlO₃, YVO₄, S-VAP, GdVO₄, GLF, BYF,KYF, KLYF, KLGF, GGG, LOS and BGO.
 3. The laser oscillator according toclaim 1, wherein said dielectric grains are doped with one oflanthanoide series, Cr and Ti as said luminous element.
 4. A laseroscillator according to claim 1, wherein the element of the periodictable is an element in the lanthanide series of the periodic table.
 5. Alaser oscillator according to claim 1, wherein the element of theperiodic table is selected from the group consisting of Ti, Yb, Cr, Nd,Er and Hf.
 6. A laser oscillator according to claim 1, wherein thedielectric grains of a diameter of 1 μm or less.
 7. A laser oscillatoraccording to claim 1, wherein the dielectric material is selected fromthe group consisting of Al₂ O₃, YAG, YAlO₃, YVO₄, S-VAP, GdVO₄, GLF,BYF, KYF, KLYF, KLGF, GGG, LOS and BGO.
 8. A laser oscillatorcomprising:a large number of dielectric grains having a first refractiveindex and being doped with an element of the periodic table which hasluminous properties; fluid having a second refractive index matched withthe first refractive index of said large number of dielectric grains,said dielectric grains being dispersed in said fluid to form asuspension; an optical resonator; and a cell positioned in said opticalresonator, in the path of light passing through said optical resonator,and holding said suspension.
 9. The laser oscillator according to claim8, further comprising a circulation system connected to said cell forcooling said suspension.
 10. The laser oscillator according to claim 9,wherein said suspension flows within said cell in a directionsubstantially parallel to an optical axis of said optical resonator. 11.The laser oscillator according to claim 9, wherein said suspension flowswithin said cell in a direction traversing an optical axis of saidoptical resonator.
 12. The laser oscillator according to claim 8,wherein said cell has a shape of slab.
 13. The laser oscillatoraccording to claim 12, wherein said cell has end faces inclined atBrewster's angle.
 14. The laser oscillator according to claim 8, whereinsaid cell has a shape of rod.
 15. A laser oscillator according to claim8, wherein the element of the periodic table is an element in thelanthanide series of the periodic table.
 16. A laser oscillatoraccording to claim 8, wherein the element of the periodic table isselected from the group consisting of Ti, Yb, Cr, Nd, Er and Hf.
 17. Alaser oscillator according to claim 8, wherein the dielectric grains ofa diameter of 1 μm or less.
 18. A laser oscillator according to claim 8,wherein the dielectric material is selected from the group consisting ofAl₂ O₃, YAG, YAlO₃, YVO₄, S-VAP, GdVO₄, GLF, BYF, KYF, KLYF, KLGF, GGG,LOS and BGO.
 19. A laser oscillator comprising:a large number ofdielectric grains having a first refractive index and being doped withan element of the periodic table which has luminous properties; fillingliquid having a second refractive index matched with the firstrefractive index of said dielectric grains, said large number ofdielectric grains being distributed and immersed in said filling liquid;an optical resonator; and a cell positioned in said optical resonator,in the path of light passing through the optical resonator, and holdingboth said dielectric grains and said filling liquid.
 20. The laseroscillator according to claim 19, further comprising a circulationsystem connected to said cell for cooling said filling liquid.
 21. Thelaser oscillator according to claim 20, wherein said filling liquidflows within said cell in a direction substantially parallel to anoptical axis of said optical resonator.
 22. The laser oscillatoraccording to claim 20, wherein said filling liquid flows within saidcell in a direction traversing an optical axis of said opticalresonator.
 23. The laser oscillator according to claim 19, wherein saidcell has a shape of slab.
 24. The laser oscillator according to claim23, wherein said cell has end faces inclined at Brewster's angle. 25.The laser oscillator according to claim 19, wherein said cell has ashape of rod.
 26. A laser oscillator according to claim 19, wherein theelement of the periodic table is an element in the lanthanide series ofthe periodic table.
 27. A laser oscillator according to claim 19,wherein the element of the periodic table is selected from the groupconsisting of Ti, Yb, Cr, Nd, Er and Hf.
 28. A laser oscillatoraccording to claim 19, wherein the dielectric grains of a diameter of 1μm or less.
 29. A laser oscillator according to claim 19, wherein thedielectric material is selected from the group consisting of Al₂ O₃,YAG, YAlO₃, YVO₄, S-VAP, GdVO₄, GLF, BYF, KYF, KLYF, KLGF, GGG, LOS andBGO.